A review paper by BTO considers 203 studies of the effects of air pollution on 231 bird species. Of these studies, 82% document at least one negative effect associated with increasing levels of pollution. The review also highlights biases towards particular study species, especially Great Tit and Pied Flycatcher, and also towards particular geographical regions (Western Europe) and pollutants (heavy metals). The paper proposes research approaches that could help to provide a fuller understanding of how birds are impacted by air pollution.

The latest review of the conservation status of birds in Wales. The report assessed all 220 bird species which regularly occur in Wales. There are now 60 species of bird on the Red List, with 91 on the Amber List and just 69 - less than a third of the total number of species - on the Green List. The latest review of the conservation status of birds in Wales comes 20 years after the first, when the Red List was less than half the length it is today. The report assessed all 220 bird species which regularly occur in Wales. There are now 60 species of bird on the Red List in Wales, with 91 on the Amber List and 69 on the Green List. The Birds of Conservation Concern in Wales report assesses the status of each species against a set of objective criteria. Data sources include the BTO/JNCC/RSPB Breeding Bird Survey and the BTO/RSPB/JNCC Wetland Bird Survey, as well as Bird Atlases and other BTO-led monitoring schemes and citizen science initiatives. These are used to quantify the changing status of the species’ Welsh population. The UK, European and global conservation status of the species is also considered, placing the Welsh population into a wider context. The Red ListSwift, Greenfinch and Rook – familiar breeding species in steep decline across the UK – are among the new additions to the Welsh Red List, which now also includes Purple Sandpiper, on account of a rapidly shrinking Welsh wintering population, and Leach’s Petrel, an enigmatic seabird in decline across its global range. These species now sit alongside well-known conservation priorities, such as Curlew, Hen Harrier and Turtle Dove as birds at risk of being lost from Wales for good. Uplands and woodlands Many of the species on the Red List are found in upland and farmland habitats. Starling, Tree Sparrow, Yellow Wagtail and Yellowhammer can no longer be found in much of Wales, while iconic species of mountain and moorland, such as Ring Ouzel, Merlin and Black Grouse, remain in serious trouble. Wales is well known for its populations of woodland birds; however, many of these – including Lesser Spotted Woodpecker, Willow Warbler and Spotted Flycatcher – also feature on the Red List. Goldcrest, which has seen its Welsh population shrink alarmingly in recent decades, is another new addition. On the coast The assessment for Birds of Conservation Concern Wales 4 took place before the impacts of avian influenza could be taken into account. Breeding seabird species have been struggling in Wales for many years, however, and most were already of conservation concern before the outbreak of this disease. Kittiwake, Puffin, Black-headed Gull, and Common, Arctic and Sandwich Tern remain on the Red List. Wales holds internationally significant numbers of breeding seabirds, making the decline of these colonies a global concern. The Amber ListDeclines in Wheatear, Garden Warbler and House Martin - all migrants which breed in Europe and winter in sub-Saharan Africa - mean these species have moved from the Green List to the Amber List. Many other ‘Afro-Palearctic' migrant species are also in decline, but the potential reasons for this, such as habitat loss and reduced availability of invertebrate prey, are not well understood. Closer to home, the declines in the Welsh Chaffinch population, linked to the disease trichomonosis, have seen the species Amber-listed. A number of other species have been placed on the Amber List because of the wider importance of their Welsh populations, which in each case make up more than half the UK total. Wales is home to more than three-quarters of the UK’s Choughs, for example, so recent declines are cause for concern. The nation’s breeding populations of Manx Shearwater, Pied Flycatcher, Goshawk and Hawfinch also account for more than half the UK total, as does its wintering population of Spotted Redshank. It’s not all bad news, though: some species now on the Amber List have moved up from the Red List, indicating some positive change in their population trends. These include Common Sandpiper, Great Black-backed Gull, Bullfinch, Goldcrest and Pied Flycatcher. The Green ListWhile the report contains much cause for alarm, several conservation success stories shine through. Red Kite was almost lost as a British bird during the first half of the 20th century, when only a handful of pairs remained in remote Welsh valleys. Since then, a sustained conservation effort has brought the species back from the brink. Wales is now home to more than 2,500 pairs of Red Kite and the species has now been moved to the Green List, reflecting this incredible change in fortunes. Song Thrush, Reed Bunting, Long-tailed Tit, Redwing and Kingfisher are among the other species to have gone Green, providing much-needed hope that things can go up as well as down.

While the study of a single population can provide important insights into what is driving population change at a local scale, we really need information from many different populations if we are to deliver effective conservation measures for declining migrant birds. This study helps to address this knowledge gap.

The increasing temperatures associated with a changing climate may disrupt ecological systems, including by affecting the timing of key events. If events within different trophic levels are affected in different ways then this can lead to what is known as phenological mismatch. But what is the evidence for trophic mismatch, and are there spatial or temporal patterns within the UK that might point to mismatch as a driver of regional declines in key insect-eating birds? A changing climate is leading to changes in the timing of key ecological events, including the timing of bud burst, the spring peak in leaf-eating caterpillar biomass and the timing of egg-laying in many bird species. If the timings of these different events shift at different rates then there is a danger that they may get out of synch with one another, something that is referred to as phenological mismatch. This may be a particular problem for birds like Blue Tit, Great Tit and Pied Flycatcher, which time their breeding attempts to exploit the spring peak in caterpillar abundance. Much of the recent work on mismatch and its impacts on the fitness and population trends of caterpillar-eating birds has looked at changes over time. However, it is also possible for mismatch to vary in space if species respond differently in different areas, perhaps because of local adaptation to geographic variation in the cues that they use. This paper looks at mismatch in both space and time, using information from three trophic levels, namely trees, caterpillars and caterpillar-eating birds. While information on bud burst came from 10,000 observations of oak first leafing for the period 1998-2016, that for caterpillar biomass was inferred from frass traps set beneath oak trees at sites across the UK for the period 2008-2016. Bird phenology data came from the ‘first egg date’ values calculated from 85,000 nest records of Blue Tit, Great Tit and Pied Flycatcher. The focus of the work was on the relationship between the phenologies of these interacting species; where timing changes more in one species than the other, this is indicative of spatial or temporal variation in the magnitude of mismatch. The results reveal that, for the average latitude (52.63°N) and year, there is a 27.6 day interval between the timing of oak first leaf and peak caterpillar biomass. With increasing latitude, the delay in oak leafing is significantly steeper than that of the caterpillar peak. At 56°N the predicted interval between these two trophic levels drops to 22 days. In the average year and at the average latitude, the first egg dates of Blue Tits and Great Tits were roughly a month earlier than peak caterpillar biomass, meaning that peak demand for hungry chicks occurred soon after the peak in resource availability. Interestingly, peak demand in Pied Flycatchers occurred nearly two weeks later than peak caterpillar availability, suggesting a substantial trophic mismatch between demand and availability for this species within the UK. However, it is worth noting that Pied Flycatchers provision their nestlings with fewer caterpillars and more winged invertebrates compared to the tit species studied, so they may be less dependent on the caterpillar peaks. The work also revealed that the timing of first egg date between years varied by less than the variation seen in timing of the caterpillar resource peak, which gave rise to year-to-year variation in the degree of mismatch. For every 10 day advance in the caterpillar peak, the corresponding advance in the three bird species is 5.0 days (Blue Tit), 5.3 days (Great Tit) and 3.4 days (Pied Flycatcher). In late springs, peak demand from the tits is expected to coincide with the peak resource availability, with flycatcher demand occurring shortly after. In early springs, the peak demand of nestlings of all three species falls substantially later than the peak, leaving the three mismatched. Warmer conditions also shortened the duration of caterpillar peaks. One of the key findings of the work is that in the average year there is little latitudinal variation in the degree of caterpillar-bird mismatch. This means that more negative declines in population trends of certain insectivorous birds in the southern UK, driven by productivity, are unlikely to have been driven by greater mismatch in the south than the north. The lack of evidence for latitudinal variation in mismatch between these bird species and their caterpillar prey suggests that mismatch is unlikely to be the driver of the spatially varying population trends found in these and related species within the UK.